1. Field of the Invention
The present invention relates to an adaptive defrost control system for an automatically defrosting refrigeration apparatus.
2. Description of the Related Art
In a conventional refrigeration apparatus, the refrigerant evaporator accumulates frost at a rate which depends on a number of conditions. These conditions include the number of times the refrigeration apparatus is accessed, the ambient humidity, and the total accumulated compressor run time. Although these conditions are variable, in the conventional non-adaptive system, the defrost cycle is initiated a fixed period of time after the previous defrost cycle has ended, regardless of the actual frost buildup.
In order to increase efficiency and thereby reduce energy costs, as well as increase the quality of products being stored in the refrigeration apparatus, it has previously been proposed to base the initiation of the defrost cycle on need, i.e., to operate the defrost heater only when frost build-up becomes excessive. Because measurement of the actual frost accumulation is difficult, it has also been proposed to initiate the defrost time based on an estimated rather than actual frost accumulation.
This type of adaptive defrost system was disclosed, for example, in U.S. Pat. No. 3,111,894, which proposed that the rate of frost accumulation be estimated based on an assumed inverse relationship between the frost accumulation and the time required for the defrost heater to raise the evaporator temperature to a predetermined temperature during a previous defrost cycle, with the period between defrosts being controlled accordingly.
The inverse relationship method of estimating frost accumulation was also used in the system described in U.S. Pat. No. 4,156,350. This patent discloses a digital timer circuit for calculating the interval between defrost cycles, rather than the heat-absorbing body and analog circuitry disclosed in the earlier U.S. Pat. No. 3,111,894, but the method used to calculate the assumed frost accumulation period is otherwise the same in both prior patents, i.e., it is based on a direct inverse relationship between the previous defrost time and the frost accumulation period or time between defrost cycles.
While the adaptive defrost control system of U.S. Pat. Nos. 3,111,894 and 4,156,350 offers improved efficiency in some situations, the assumed inverse relationship is not necessarily optimal. For example, in situations where successive defrost times fluctuate significantly, the sign of the change in the frost accumulation period will lag the actual change, and the assumed and actual frost accumulation periods will thus rarely converge, resulting in an interval between defrosts which is shorter or longer than necessary.
To solve this problem, the interval between defrost cycles could be based on the inverse of an average of more than one previous defrost time, rather than on the inverse of a single previous defrost time. While this would reduce the effect of widely fluctuating defrost times, the resulting prediction would still not be optimal, as illustrated by the following example:
EXAMPLE OF WHY INVERSE RELATIONSHIP IS NOT OPTIMAL EVEN WHERE PREVIOUS TIMES ARE AVERAGED
If one assumes the following inverse relationship between the defrost time (dt) and the time between defrost operations (tbf): EQU dt=9 minutes.fwdarw.tbf=12 hours EQU dt=10 minutes.fwdarw.tbf=11 hours EQU dt=11 minutes.fwdarw.tbf=10 hours EQU dt=12 minutes.fwdarw.tbf=9 hours,
then for the situation in which the last three defrost times, in order, beginning with the earliest defrost time, change as follows (for example, due to a season change in ambient humidity): EQU dt=12 minutes EQU dt=11 minutes EQU dt=10 minutes.
Using just the last defrost time would give a time between defrosts of EQU tbf=11 hours,
while using the average of the last three defrost times would give a time between defrosts of EQU tbf=10 hours.
The latter result would clearly be contrary to the trend of decreasing defrost times (12 min..fwdarw.11 min..fwdarw.10 min.) Consequently, using the average of the previous defrost times as the basis for the inverse relationship would actually give a worse result that just using the last defrost time, while using the last defrost time would also be inaccurate if the clear trend of decreasing frost accumulation were to continue.
In order to solve this problem, a new system would be desirable which takes into account the direction as well as the magnitude of changes in the previous defrost time.
In order to further improve the predictive accuracy of an adaptive defrost system, it would also be desirable to depart from the strict inverse relationship concept of the systems described in U.S. Pat. Nos. 3,111,894 and 4,145,350 by monitoring the compressor run cycles during a frost accumulation period and evaluating the run times based on a variable standard which takes into account the trends in the defrost interval. While U.S. Pat. No. 4,156,350 discloses monitoring of a total compressor run time during a frost accumulation period, individual cycles in the prior system are not compared to a variable standard for optimal efficiency.